Starting with a pairwise, spatially and orientationally dependent intermolecular potential constructed to include steric effects, we carry out a systematic solution of the mean-field equation for liquid crystals. A model parameter which is connected to the molecular structure measures the strength of the steric forces. Its inclusion makes possible a semiquantitative comparison of our results to the experimentally obtained phase diagrams of several homologous series. The model predicts phase diagrams similar to that found by Lee et al. Improvements over the latter include (1) the characteristic feature of the smectic $A$ phase, that the director prefers to be perpendicular to the smectic layers, arises naturally from our model; (2) the connection of one of our model parameters with the structure of molecules can now be used to explain the differences in phase-transition properties between homologous series whose molecules are of similar structure but differ in the length of the rigid section.

• Received 31 May 1977

DOI:https://doi.org/10.1103/PhysRevA.17.1529